A series of four enzymes having five activities are required in M. luteus to complete the excision repair of UV-irradiated DNA. This repair cycle consists of an incision step catalyzed by a UV-specific endonuclease whose specificity for the types of DNA conformational changes is to be examined in detail with phi X174 RFI circular duplexes. The mechanism and characterization of such an enzyme from E. coli is to be studied as well. The excision step is catalyzed by an exonuclease whose mechanism and specificity is to be assessed with model polymers, Excised DNA is subjected to a prereinsertion step catalyzed by a 3' yields 5' exonuclease associated with a DNA polymerase from M. luteus. Its mechanism is to be examined as a component of the reinsertion step. The relationship of these enzymatic studies to repair of phi 174 RFI transfecting activity will be studied.